Saturday, March 21, 2009

How personal trainers in Austin calculate metabolism

Tim Richardson

What are basal and resting metabolic rates (RMR)?

These two terms are used interchangeably, although they are not technically the same. Resting metabolic rate is really what most lay people in Austin mean when they say basal metabolic rate, and I talk here only about resting metabolic rate (RMR). Basal metabolic rate is a precise calculation with a precise definition; RMR is close enough for practical purposes. Resting metabolic rate is the energy required by an animal to stay alive with no activity. Therefore, your real metabolic rate is always significantly higher than your RMR. Calculating RMR is a very useful first step in calculating your real metabolic rate.

Your metabolic rate = your resting metabolic rate (easy to calculate reasonably accurately) + energy consumed by your daily activities (must guestimate).

What determines Resting Metabolic Rate?

A very small number of people have physical conditions that give them strange resting metabolic rates. However, for the vast majority of people in Austin, resting metabolic rate can be calculated knowing a few key variables. They are age, sex, weight, height and fat-free body mass. Fat-free mass is a very important variable. Weight and height are used in one formula to determine body surface area.

When does the body change Resting Metabolic Rate? Does cutting your food intake reduce resting metabolic rate?

The body CAN NOT change resting metabolic rate per unit of fat-free body mass. Studies have shown this. For an article that is on-line in full text by a well-known researchers in the field, see Genetic Influences on the Response of Body Fat and Fat Distribution to Positive and Negative Energy Balances in Human Identical Twins, Claude Bouchard and Angelo Tremblay
Your resting metabolic rate will decrease if you lose muscle, and increase if you gain muscle. Losing fat alone will not lower your RMR (and note that you will need to follow a very sensible program probably including weight training to lose fat without losing muscle). You have probably heard that people who go on crash diets end up lowering their metabolic rate, which means when they go off the diet, they put on fat more easily than before they started. Because they have lost muscle, they have lowered their metabolic rate. However, the amount of energy burnt per unit of fat-free weight does not change; poor dieters end up with fewer units of fat-free weight, and that's where their vicious cycle comes from.

Are some people's digestive systems more efficient than others?

No (except perhaps some people with disorders). And your system does not become more or less efficient in response to changing food intake. See the above study. Even obese people rarely have more efficient bodies. Researchers inspect the energy value of faeces to determine this.

Given the same values for the variables, how much does resting metabolic rate vary between people?

In other words, what is the error in the formulas used to calculate RMR? The latest research indicates there is a low variance in RESTING metabolic rate between individuals who have the same values for the key variables. That is, given someone's age, their fat free mass, their height and their sex, the formulas are accurate.
"Recent evidence thus supports the conclusion that within-subject variations in BMR [more or less the same as RMR] are small and insignificant, even when energy intake and physical activity are uncontrolled, (Shetty & Soares, 1988). This effectively refutes the Sukhatme-Margen hypothesis." (See link at the end of this page).

Formulas for calculating metabolic rate

Apart from the formulas here, you can also use what is the best calculator on the web (that I've seen): Follow the link to the Healthy Body Calculator.You don't need to fill in all the information to get your metabolic rate. This is your total estimated daily metabolic rate, not your resting metabolic rate.

Formulas from "Advanced Fitness Assessment and Exercise Prescription", (4th ed) , Dr V. H. Heyward. pub Human Kinetics and available at Amazon (I own the third edition).
Their formulas only calculate Resting Metabolic Rate! There is a second step after this: calculating your energy use due to your daily activities.

Output is in kcal (1000 calories; nutritional information is nearly always given in kcal even when it says calories. It is often also shown in joules or kilojoules, which is different).

Harris-Benedict equations
BW = body weight in kilograms, HT = height in cm, Age in years
Men: RMR = 66.473 + 13.751*BW + 5.0033*HT - 6.755*Age
Women: RMR = 655.0955 + 9.463*BW + 1.8496*HT - 4.6756*Age

Fat Free Mass formula
This is the same for men and women.
RMR = 1.3 * FFM * 24
FFM is your body weight - your body fat in kg. You need to know your body fat percentage to work this out. If you weight 100kg and your body fat percentage is 10%, then your fat free mass is 100 - 100*10% = 90%. 10% is for a man pretty normal. If you are fat, then you should work with a number between 15% and 25% (better yet, measure it, don't guess).

Quick estimate formula
Men: BW (in kg) * 24.2
Women: BW (in kg) * 22.0

Note that only one formula uses fat free mass, even though this is a key determinant (so the other formulas are tuned. Calculate your RMR with all three methods, and take an average, unless you are obese. Then you should pay more attention to the Fat Free Mass equation. Work out a plus and minus amount using the three formulas. If the first method tells you 1720 kcal, the second 1750 kcal and the third 1760 kcal, then you would say that your RMR is about 1743 +- 20